pwm: vt8500: Rename variable pointing to driver private data

Status quo is that variables of type struct vt8500_chip * are named
"vt8500", "chip". Because usually only struct pwm_device * variables
are named "pwm" and "chip" is usually used for variabled of type
struct pwm_chip *.

So consistently use the same and non-conflicting name "vt8500".

Signed-off-by: zhaoxiao <zhaoxiao@uniontech.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>